1. Field of the Invention
The present invention relates to a level shift circuit, and more particularly to a signal level shifting circuit that is able to turn off a latching circuit in the signal level shifting circuit during a signal level transition period.
2. Description of the Prior Art
The voltage level shifting circuit is one of the most important devices in a liquid crystal display (LCD). The voltage level shifting circuit is utilized to shift a digital signal with relative low voltage level into a digital signal with relative high voltage level to drive the driving circuit in the control system of the liquid crystal display, wherein the driving circuit is operated under the relative high voltage level. Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional voltage level shifting circuit 100. The voltage level shifting circuit 100 comprises a differential input pair cascoded to a cross-coupled pair, wherein the differential input pair comprises N type transistors M1, M2, and the cross-coupled pair comprises P type transistors M3, M4. Furthermore, the cross-coupled pair is connected to a supply voltage VDD1, and the differential input pair is connected to a ground voltage VSS as shown in FIG. 1. Furthermore, the voltage level shifting circuit 100 further comprises an inverter 102, which is utilized to generate an input signal VIN2 that has an inversed phase corresponding to the input signal VIN1. The input signal VIN1 is inputted to a gate terminal N1 of the N type transistor M1, and input signal VIN2 is inputted to a gate terminal N2 of the N type transistor M2. Furthermore, the inverter 102 is operated between a supply voltage VDD2 and the ground voltage VSS, wherein the supply voltage VDD1 is higher than the supply voltage VDD2. Accordingly, when the input signal VIN1 is a low voltage level signal, i.e., the ground voltage VSS, the input signal VIN2 is a high voltage level signal, i.e., the supply voltage VDD2, and vice versa. Then, by appropriately modifying the sizes of the N type transistors M1, M2 and the sizes of the P type transistors M3, M4, the voltage level shifting circuit 100 shifts the relative low voltage of the input signals VIN1, VIN2 to the relative high voltage of the output signals VO1, VO2, and outputs the output signals VO1, VO2 at the output terminals NO1, NO2 respectively. More specifically, when the input voltage VIN1 is the ground voltage VSS, and the input voltage VIN2 at the gate terminal N2 is the supply voltage VDD2, then the N type transistor M1 is turned off and the N type transistor M2 is turned on. On the other hand, the P type transistor M3 is turned on, and the P type transistor M4 is turned off. Therefore, the output signal VO1 at the output terminal NO1 is latched to the supply voltage VDD1, and the output signal VO2 at the output terminal NO2 is latched to the ground voltage VSS, and vice versa. However, since the supply voltage VDD1 is higher than the supply voltage VDD2, the N type transistors M1, M2 should be designed to have a large width to increase the turn on current of the N type transistors M1, M2, and the P type transistors M3, M4 should be designed to have a longer length than the length of the N type transistors M1, M2. In addition, since there are a large number of voltage level shifting circuits 100 being used in a driving system of the LCD, the conventional voltage level shifting circuit 100 may occupy a large area in the control system of the LCD, and this consequently increases the cost of the LCD. Therefore, how to reduce the size of a voltage level shifting circuit meanwhile to effectively drive a liquid crystal displaying unit is an urgent problem in the field of LCD.